Gasification of Carbonaceous Material

ABSTRACT

A process for gasifying a carbonaceous particulate feedstock includes dividing the carbonaceous particulate feedstock into at least two feedstock fractions, each feedstock fraction including particulate material within a predetermined particle size range so that there are at least a smaller particle size feedstock and a larger particle size feedstock. The smaller particle size feedstock is fed to at least one gasifier and the larger particle size feedstock is fed to at least one other gasifier.

THIS INVENTION relates to gasification of carbonaceous material. In particular, it relates to a process for gasifying a particulate carbonaceous feedstock.

The use of gasifiers for the conversion of carbonaceous feedstocks, usually and predominantly coal, to crude gas which includes steam, carbon dioxide, hydrogen, carbon monoxide, methane and a broad range of hydrocarbons such as heavy oils, tars and pitches, is well known. One example of such a gasifier is a fixed bed dry bottom gasifier (also known as a moving bed dry ash gasifier). These gasifiers can make use of lump coal with a typical particle size distribution range from about 4 mm or 8 mm up to about 70 mm or 100 mm. Advantageously, these gasifiers do not require much feedstock preparation. Typically, particle top size is determined by a crusher opening of a crusher used for crushing the feedstock, while the particle bottom size is determined by a bottom screen aperture in a screening plant, which is typically located before a gasification plant.

Although the aforementioned gasification technology is robust and able to handle a wide and variable range of feedstocks, operation can be improved by careful management of particle size distribution of the feedstock and the avoidance of out-of-seam or non-carbonaceous matter. Although some of the non-carbonaceous matter can be removed in an upgrading or beneficiation stage, such a stage adds to the overall capital and operating costs of a gasification process. A portion of the usable carbonaceous material is lost with material discarded from the upgrading stage, and the discarded material has to be disposed of in an environmentally acceptable manner to prevent spontaneous combustion and ground water contamination due to acid rock drainage.

According to one aspect of the invention, there is provided a process for gasifying a carbonaceous particulate feedstock, the process including

dividing the carbonaceous particulate feedstock into at least two feedstock fractions, each feedstock fraction including particulate material within a predetermined particle size range so that there are at least a smaller particle size feedstock and a larger particle size feedstock; and

feeding the smaller particle size feedstock to at least one gasifier and the larger particle size feedstock to at least one other gasifier.

Thus, typically, in accordance with the invention a gasifier receives only one of the feedstocks and typically none of the gasifiers receives more than one of the feedstocks. The carbonaceous particulate feedstock is typically coal.

As will be appreciated, each feedstock fraction has a narrower particle size distribution range than the particulate carbonaceous feedstock from which the feedstock fractions have been derived.

The smaller particle size feedstock may have a particle size distribution with an upper limit which is at least about 5 times its lower limit, preferably at least about 6 times its lower limit, more preferably at least about 7 times its lower limit, e.g. between about 8 and about 9 times its lower limit. Lower and upper limits are typically determined by screen sizes used to prepare the two or more feedstocks each with a predetermined particle size range.

The larger particle size feedstock may have a particle size distribution with an upper limit which is at least about 1.5 times its lower limit, preferably at least about twice its lower limit, more preferably at least about 2.5 times its lower limit, e.g. between about 2.6 and about 3 times its lower limit.

The aforementioned particle size distribution ranges are thus substantially narrower than the particle size distribution range of a typical conventional coal feedstock, in which the upper particle size limit is typically at least about 12 times, e.g. about 15 times the lower particle size limit of the range.

As will be appreciated, when two feedstocks are used, the upper limit of the particle size distribution range of the smaller particle size feedstock is typically about the same as the lower limit of the particle size distribution range of the larger particle size feedstock. This particle size, which thus lies between the lower limit of the particle size distribution range of the smaller particle size feedstock and the upper limit of the particle size distribution range of the larger particle size feedstock, depends on a number of factors, such as the effect of the mining method on the particle size distribution of run-of-mine coal, the mechanical strength of the coal deposit and the number of gasifiers which have to be supplied with each feedstock fraction.

Preferably, the smaller particle size feedstock is fed to a gasifier or gasifiers at a mass flow rate at least about twice, more preferably at least about 2.5 times, even more preferably at least about 2.75 times, e.g. about 3 times the mass flow rate at which the larger particle size feedstock is fed to a gasifier or gasifiers.

The larger particle size feedstock may include more non-carbonaceous material on a percentage by mass basis, such as out-of-seam matter, than the smaller particle size feedstock. Non-carbonaceous material is often present in a coal feedstock due to accidental extraction of the roof and the floor of the coal seam during mining, or because a coal deposit contains mineral intrusions inside the seam which are thus inadvertently extracted with the coal.

The process may include subjecting at least the larger particle size feedstock to a beneficiation or an upgrading stage. The upgrading stage may include removing at least a portion of the non-carbonaceous material from the larger particle size feedstock. The upgrading stage may include a dense medium separation step.

The upgrading stage may include a size reduction step in which the average particle size of the particulate carbonaceous material of the larger particle size feedstock is reduced.

The gasifier or gasifiers fed with the smaller particle size feedstock may be operated with an oxygen to pure gas volumetric ratio of between 0.19 and 0.21 and coal as feedstock and may produce a raw gas comprising between 26 and 28 mole % CO₂, preferably between 27 and 28 mole % CO₂ at a pure gas yield of at least 1640 Nm³/ton of dry ash free coal (DAF coal), preferably at least 1660 Nm³/ton of dry ash free coal. The pure gas yield may be delivered at a standard deviation of not more than 28, preferably not more than 17 Nm³/ton of dry ash free coal.

The invention will now be described and illustrated, by way of example only, by means of the accompanying drawings and the results of tests conducted to show the advantages of employing the process of the invention.

In the drawings,

FIG. 1 shows a graph of coal feedstock stone content;

FIG. 2 graphically illustrates feasible areas of an operating regime for a gasifier receiving a feedstock with a broad particle size distribution of 70×4 mm;

FIG. 3 graphically illustrates feasible areas of an operating regime for a gasifier receiving a feedstock with a narrow particle size distribution of 35×4 mm;

FIG. 4 graphically illustrates feasible areas of an operating regime for a gasifier receiving a feedstock with a narrow particle size distribution of 35×4 mm, with stone content as one parameter;

FIG. 5 shows a graph of oxygen to pure gas ratios versus raw gas CO₂ concentration for reference tests and invention tests;

FIG. 6 shows a graph of pure gas yield versus raw gas CO₂ concentration in raw gas for the reference tests and invention tests;

FIG. 7 shows a graph of pure gas yield versus stone content of the coal feedstock for the reference tests and invention tests; and

FIG. 8 shows a graph of gasifier bed pressure drop versus oxygen load for the reference tests and invention tests.

GASIFIER TESTS ILLUSTRATING ADVANTAGES OF THE INVENTION

Coal Preparation

All invention tests in a test gasifier were conducted with a coal blend produced by six mines in South Africa. The sources were blended by means of Stacker & Reclaimers, following the same method, standard and level of homogenization as the normal coal feed to a commercial gasification plant.

The blend used for these tests was therefore representative of the normal feedstock and is referred to as the “standard blend”. The invention tests are compared to reference tests which were also done with the standard blend feedstock.

The final particle size distribution of the coal fed to the test gasifier was obtained by a process of dry and wet screening. Run-of-mine coal was first dry screened with a 35 mm screen, an overflow providing a coarse (100×35 mm) fraction or test material, and an underflow of the screen (−35 mm) providing a fine fraction or test material. The second step in the preparation was to screen the −35 mm material in order to remove the −4 mm fines and size the fine test material (35×4 mm). This polishing step was done by means of a wet screening process. Multiple size distributions can be prepared in a single screening unit equipped with multiple screens.

The coarse test material was handled and stockpiled by means of machines which generated some fine material. The coarse test material was also polished by means of a wet screening process to remove the −10 mm material.

For all the invention test runs the coal was used well within the predetermined time limits regarding ageing of the coal to prevent degradation.

Test Gasifier Operation

The test gasifier was operated manually according to a statistically designed test programme. Some of the most critical measurements, including gas liquor flow and CO₂ content of the raw gas, were verified with additional measurements. Reliable crude gas composition determinations were made through on-line analysers and frequent hand samples. This data was used for determining mass balances.

All relevant data was recorded through a dedicated computer system operating independently from the plant's process instrumentation system. Mass balance calculations and interpretations were based on this data. All raw data and calculated data were recorded on a database for evaluation purposes.

Coal, gas, tar, gas liquor and ash samples were taken regularly. Analyses were carried out by in-house laboratories as well as by credible commercial laboratories. All the other values were measured continuously using dedicated on-line instrumentation.

Test Program

Reference Tests

The reference tests were done under the following conditions: Coal: Standard blend feedstock Particle size distribution: 100 × 5 mm from Wet Screening Plant Gasifier: Sasol-Lurgi Mark IV.

A load condition of approximately 10 kNm³/h to 13 kNm³/h oxygen was aimed for. The CO₂ in raw gas concentrations aimed for were ±26% and ±28% CO₂ in the dry raw gas. The test schedule which was executed for the reference tests is given in Table 1. TABLE 1 TEST SCHEDULE EXECUTED FOR REFERENCE TESTS Target oxygen load in Target Test kNm³/h % CO₂ in raw gas Reference test A 9.5 28.0 Reference test B 9.5 26.5 Reference test C 11.5 26.5

The test schedule for the invention tests is given in Table 2. TABLE 2 TEST SCHEDULE EXECUTED FOR INVENTION TESTS Particle size Target oxygen load in Target fraction Test kNm³/h % CO₂ in raw gas  35-4 mm 32A 9.5 28.0 32B 9.5 26.5 32C 13.5 26.5 32D 13.0 28.0 35-70 mm 33A 9.5 28.0 33B 9.5 26.5 33C 12.0 27.0 Test Results Coal Characteristics

In general the average coal properties of the 35×4 mm coal blend were very similar to that of standard blends used for the reference tests. Some properties of the 35×70 mm fraction differed from the standard coal blend. In particular, it was found that the ash content was significantly higher, which indicates that the inorganic material has preference for the courser fraction.

With reference to FIG. 1, the stone content (defined as the sink fraction at a relative density=1.95) graphically explains the higher ash content of the 35×70 mm coal fraction. The stone content concentrated in the coarse fraction to ±24%, which is more than double the amount of stone in the standard coal blend. The stone content of the 35×70 mm fraction also showed a much larger variation than the stone content of the 35×4 mm fraction and the previous reference tests. In the invention tests the coarse fraction was not beneficiated or crushed down, but was used as is for gasification coal feed.

No significant differences were noted in the ash melting temperatures of the 35×4 mm and 35×70 mm coal fractions which were similar to that of previous reference tests.

Operational Gasifier Stability

All invention tests were completed with relative stable gasifier operation. Instabilities experienced were due to common mechanical difficulties.

Data Evaluation

In this section the 35×4 mm and 35×70 mm coal fraction invention test results are compared to those of the reference tests.

The following process parameters were compared: Utility consumption: Total steam, oxygen Product yields: ${Pure}\quad{{gas}\quad\left\lbrack \frac{{Dry}\quad{raw}\quad{{gas} \times \begin{pmatrix} {100 -} \\ {{{mole}\quad\%\quad{CO}_{2}} -} \\ {{mole}\quad\%\quad H_{2}S} \end{pmatrix}}}{100} \right\rbrack}$ per ton of dry ash free coal (PG/t DAF) Bed pressure drop: Pressure drop over the gasifier bed

In 1998 Sasol decided to isolate one fixed bed dry bottom gasifier at its Secunda site as a test gasifier. A total of thirty-one comprehensive tests have been conducted on the test gasifier from September 1998 to March 2000. These tests will be referred to as historical tests. The historical tests were designed and performed according to a statistical experimental program. The process variables investigated and used in this factorial experimental design were stone content, coal top and bottom particle size, oxygen load (oxygen feed rate) and CO₂ in raw gas concentration. Statistical models were constructed on the data from the historical tests for predicting gasifier performance at specific values of the process variables.

In addition, statistical models were developed and applied for determining desirable operating regimes by means of statistical robustness studies. FIG. 3 demonstrates the improved robustness of the gasifier when operated with a 35×4 mm coal particle size fraction compared to a typical broad particle fraction (70×4 mm) of the standard coal blend (FIG. 2). FIG. 3 depicts the feasible areas of operating regimes according to specific criteria for pure gas (PG) yield and the standard deviation (SD) of PG yield. The standard deviation indicates the variation in PG yield production due to gasifier instability. The smaller the standard deviation the more stable or robust PG yield is produced. The following are observed:

-   For the 35×4 mm fraction a very large feasible area is observed at     middle to high loads. Thus, the operability regime is expanded for     the 35×4 mm fraction. Also, the standard deviation was decreased. -   It could therefore be concluded that the most stable production of     high PG yield is obtained for coal with a 35×4 mm fraction. Notice     that these conditions provide stable production of high PG yield     irrespective of the amount of stone present in the coal, up to 10%     stone which was the maximum value set for the factorial experimental     design (FIG. 4).

The invention tests were compared to the reference tests based on the predictions obtained with the statistical models. Predictions with the statistical models could not be performed for the 35×70 mm fraction since this size fraction was not included and tested in the historical tests. Weighted averages of PG yield and utility consumption were calculated for the combined 35×4 mm and 35×70 mm fraction tests and compared with the predictions for the reference tests (70×4 mm). When the particle size distribution of the standard feedstock was screened into the 35×4 mm and 35×70 mm fractions, it was found that 75% of the material reported to the 35×4 mm fraction and only 25% reported to the 35×70 mm fraction. Therefore, when the weighted average was calculated, the following formula was used: Average of parameter=75%×(predicted value of parameter for 35×4 mm fraction)+25%×(actual value of parameter for 35×70 mm fraction) Oxygen Consumption as Indicated by O₂/Pure Gas Ratio

A graph for the O₂/pure gas ratios is shown in FIG. 5. The ratios for both the 35×4 mm and 35×70 mm coal fractions are within the normal scatter of the historical data. The invention tests demonstrate that, surprisingly, significant higher oxygen consumption was not required for the coarse fraction (35×70 mm), as was expected.

Pure Gas Yield

General trends observed for pure gas yields are shown in FIGS. 6 and 7. Because of the interactions between the parameters that determine pure gas yield, care should be taken when drawing conclusions from 2-dimensional graphs as shown in FIGS. 6 and 7.

FIG. 6 indicates that the 35×70 mm fraction gave lower pure gas yields than the reference tests with the normal broad particle size distribution, and the yields for the 35×4 mm fraction is higher than the reference test values.

Although the 35×70 mm coal fraction gave lower pure gas yields, the stone content trend in FIG. 7 indicates that the pure gas yields are higher than expected. It is clear that the pure gas yield for the 35×70 mm fraction is higher than anticipated. It is believed that the higher than expected pure gas yield for the 35×70 mm fraction is a result of the benefit of the narrower particle size distribution. This result is a surprising and unexpected advantage of this invention.

An increase of 2.5 to 3.0% in pure gas yields is expected by running the gasification process on split feed particle sizes in accordance with the invention. Although these improvements are not statistically significant on the 90% confidence interval, these improvements are large enough to be of practical importance. The improvement in pure gas yield is attributed to better C conversion (i.e. less C in ash losses), but it should be noted that the effect of particle size distribution changes on tar production, and therefore ultimately on pure gas yield, could not be quantified with the current hardware installed on the test gasifier.

Bed Pressure Drop

General trends for pressure drop over the gasifier bed were plotted to establish if unacceptably high ΔP's are obtained with the finer coal feed fraction. From FIG. 8 it can be seen that the ΔP's obtained with the 35×4 mm blends are slightly higher in some cases in comparison with historical data obtained with the normal coal blend, but not high enough to be a concern.

CONCLUSIONS

-   The average coal properties of the 35×4 mm coal blend were very     similar to that of standard blends used for the reference tests.     Some properties of the 35×70 mm fraction differ from the standard     coal blend. -   The stone content explains the higher ash content of the 35×70 mm     coal fraction. The stone content concentrated in the coarse fraction     to ±24%, which is more than double the amount of stone in the     standard coal blend and also showed a much larger variation than the     stone content of the 35×4 mm fraction and the previous reference     tests. As the 35×70 mm coal fraction is however much smaller than     the 35×4 mm coal fraction, the capital cost of equipment provided     only to beneficiate the larger fraction will be less than in     conventional processes treating the total coal feed whilst providing     a similar overall benefit. -   The operability regime is expanded for the 35×4 mm fraction. Also,     the standard deviation was decreased. -   The most stable production of high PG yield is obtained for coal     with a 35×4 mm fraction. -   Significant higher oxygen consumption was not required for the     coarse fraction (35×70 mm), contrary to expectations. -   The 35×70 mm fraction gave lower pure gas yields than the reference     tests with the normal broad particle size distribution, and the     yields for the 35×4 mm fraction are higher than the reference test     values. -   An increase of 2.5 to 3.0% in pure gas yields is expected by running     the gasification process on split feed particle sizes. Although     these improvements are not statistically significant on the 90%     confidence interval, these improvements are large enough to be of     practical importance. The improvement in pure gas yield is     attributed to better C conversion (i.e. less C in ash losses), but     it should be noted that the effect of particle size distribution     changes on tar production, and therefore ultimately on pure gas     yield, could not be quantified with the current hardware installed     on the test gasifier. -   It was observed that the ΔP's obtained with the 35×4 mm blends are     slightly higher in some cases in comparison with historical data     obtained with the normal coal blend, but not high enough to be a     concern. 

1-13. (canceled)
 14. A process for gasifying a carbonaceous particulate feedstock, the process including dividing the carbonaceous particulate feedstock into at least two feedstock fractions, each feedstock fraction including particulate material within a predetermined particle size range so that there are at least a smaller particle size feedstock and a larger particle size feedstock; and feeding the smaller particle size feedstock to at least one fixed bed dry bottom gasifier and the larger particle size feedstock to at least one other fixed bed dry bottom gasifier.
 15. The process as claimed in claim 14, in which each gasifier receives only one of the feedstocks and none of the gasifiers receive more than one of the feedstocks.
 16. The process as claimed in claim 14, in which the smaller particle size feedstock has a particle size distribution with an upper limit which is at least about 5 times its lower limit.
 17. The process as claimed in claim 16, in which the upper limit is at least 6 times the lower limit.
 18. The process as claimed in claim 17, in which the upper limit is at least 7 times the lower limit.
 19. The process as claimed in claim 14, in which the larger particle size feedstock has a particle size distribution with an upper limit which is at least about 1.5 times its lower limit.
 20. The process as claimed in claim 19, in which the upper limit is at least twice the lower limit.
 21. The process as claimed in claim 20, in which the upper limit is at least 2.5 times the lower limit.
 22. The process as claimed in claim 14, in which the smaller particle size feedstock is fed to a gasifier or gasifiers at a mass flow rate at least about twice the mass flow rate at which the larger particle size feedstock is fed to a gasifier or gasifiers.
 23. The process as claimed in claim 14, in which the larger particle size feedstock includes more non-carbonaceous material on a percentage by mass basis than the smaller particle size feedstock, the process including subjecting at least the larger particle size feedstock to a beneficiation or an upgrading stage.
 24. The process as claimed in claim 23, in which the upgrading stage includes a size reduction step in which the average particle size of the particulate carbonaceous material of the larger particle size feedstock is reduced.
 25. The process as claimed in claim 14, in which the gasifier or gasifiers fed with the smaller particle size feedstock is/are operated with an oxygen to pure gas volumetric ratio of between 0.19 and 0.21 and coal as feedstock and produce(s) a raw gas comprising between 27 and 28 mole % C0₂ at a pure gas yield of at least 1660 Nm³/ton of dry ash free coal.
 26. The process as claimed in claim 14, in which the smaller particle size feedstock has a particle size distribution with a lower limit larger than 4 mm. 